SARS-CoV-2 cell-to-cell infection is resistant to neutralizing antibodies

The COVID-19 pandemic caused by SARS-CoV-2 has posed a global threat to human lives and economics. One of the best ways to determine protection against the infection is to quantify the neutralizing activity of serum antibodies. Multiple assays have been developed to validate SARS-CoV-2 neutralization; most of them utilized lentiviral or vesicular stomatitis virus-based particles pseudotyped with the spike (S) protein, making them safe and acceptable to work with in many labs. However, these systems are only capable of measuring infection with purified particles. This study has developed a pseudoviral assay with replication-dependent reporter vectors that can accurately quantify the level of infection directly from the virus producing cell to the permissive target cell. Comparative analysis of cell-free and cell-to-cell infection revealed that the neutralizing activity of convalescent sera was more than tenfold lower in cell cocultures than in the cell-free mode of infection. As the pseudoviral system could not properly model the mechanisms of SARS-CoV-2 transmission, similar experiments were performed with replication-competent coronavirus, which detected nearly complete SARS-CoV-2 cell-to-cell infection resistance to neutralization by convalescent sera. Based on available studies, this is the first attempt to quantitatively measure SARS-CoV-2 cell-to-cell infection, for which the mechanisms are largely unknown. The findings suggest that this route of SARS-CoV-2 transmission could be of great importance for treatment and prevention of COVID-19. ImportanceImmune surveillance of viral or bacterial infections is largely mediated by neutralizing antibodies. Antibodies against the SARS-CoV-2 spike protein are produced after vaccination or infection, but their titers only partly reflect the degree of protection against infection. To identify protective antibodies, a neutralization test with replicating viruses or pseudoviruses (PVs) is required. This study developed lentiviral-based PV neutralization assays that, unlike similar systems reported earlier, enable quantitative measurement of SARS-CoV-2 neutralization in cell cocultures. Using both PVs and replication-competent virus, it was demonstrated that SARS-CoV-2 cell-to-cell infection is considerably more resistant to serum neutralization than infection with purified viral particles. The tests are easy to set up in many labs, and are believed to be more informative for monitoring SARS-CoV-2 collective immunity or entry inhibitor screening.